The present invention relates in general to apparatus for, and method of, fluidly connecting a boiler into a pressurized steam feed line, and more particularly, to a combined-cycle steam generator power plant embodying such apparatus and method. For illustration, the invention will be described in the context of a combined-cycle steam generator power plant, and for this purpose U.S. Pat. Nos. 3,955,358 and 3,965,675 of L. F. Martz et al. are hereby incorporated by reference.
In a combined-cycle electric power plant, a steam turbine producing megawatts is combined with a gas turbine also producing megawatts, and the hot exhaust gases from the gas turbine, which otherwise would be lost to atmosphere, are used to heat the circulated condensate from the condenser of the steam turbine and to generate superheated steam to drive the steam turbine.
Heat recovery steam generators, e.g., boilers, are designed to extract heat from hot gases, and to convert with such heat, water into superheated steam at a relatively high pressure. At least two such boilers have been incorporated into a combined-cycle electric power generating plant including both gas and steam turbines wherein the exhaust gases of at least two gas turbines are used to heat condensate and convert it into steam for a common steam turbine.
The combined-cycle plants have been designed in the past to have at least two steam-producing boiler-and-gas turbine units associated with one steam turbine in order to maximize the supply of heat in the conversion of heat into steam, and also for versatility in control and for maximized efficiency. A problem arises, though, with such systems, which is to be able to start and couple an inactive boiler onto a boiler already actively generating steam at normal temperature and pressure to the steam turbine. It is desirable to be able to control the start of the added boiler, i.e. which is cold, so that it reaches steam quality and flow level under efficient steam production conditions and as quickly as possible after the decision has been made to add such another boiler. This should be done without changing the operative conditions of the boiler already coupled with the steam turbine power supply line. In particular, with a gas turbine there is a fixed acceleration time, since firing of the gas turbine is independent from boiler control. Within the given start-up time for both the gas turbine and the boiler, ideally, the characteristics for the gas turbine exhaust and for air and BTU flows must be properly related. Especially, there must not be any slowdown in the start-up of the gas turbine in order to accommodate the generation of steam in relation to proper coupling conditions, otherwise this would mean BTU losses.
In the prior art, coupling with the steam supply line for the turbine is effected when the latter is already under steam supply from another boiler, by merely opening a stop valve in derivation to atmosphere and waiting until the pressure build-up from the additional boiler matches the operative pressure in the feed line, at which time the dump valve is closed and the throttle valve to the supply line is opened.
U.S. Pat. No. 4,068,475 to M. H. Binstock shows start-up control of a boiler through drain flow control in order to balance boiler inflow and outflow. This technique there disclosed, however, does not involve coupling an additional boiler into a steam system already in operation.